Add support for multipart/form-data in RIE

I would love the ability to use multipart/form-data in local testing environments for my lambda functions. The official AWS API-Gateway supports this feature so I see no reason why this package shouldn't.

It could be enabled with a --flag on the startup call or something if need be.

先追蹤 RIE 如何處理 Lambda HTTP 請求和啟動旗標，然後將 multipart/form-data 的預期行為與 AWS API Gateway 進行比較。完成的標準是本機測試支援 Lambda 函式使用 multipart/form-data，並記錄任何必要的啟動設定。

由索引模型根據 Issue 內容生成。